Many known piston pumps such as that disclosed in U.S. Pat. No. 5,975,360 to Ophardt, issued Nov. 2, 1999, have a piston which is coaxially slidable in a chamber with engagement between a radially outwardly extending disc on the piston and a wall of the chamber forming a seal which prevents fluid flow between the disc and the chamber wall inwardly and/or outwardly for proper operation of the piston. Many known such pumps suffer the disadvantage that the extent to which such a seal prevents fluid flow therepast is a function of the relative diameter of the disc and the chamber in which the disc is received as well as the inherent resiliency of the disc. The present applicants have appreciated the disadvantage that while a disc may upon manufacture have an adequate inherent bias into engagement with a chamber wall to prevent fluid flow therepast that, over time, the compression of such a disc in the chamber results in the material such as plastic forming the disc developing a set which reduces the inherent bias by which the disc is biased outwardly into engagement with the chamber wall increasing the risk of leakage past the seal. Providing a disc which has a strong inherent bias to engage the chamber wall has the disadvantage of increasing the forces required to move the piston. Additionally, with use of the piston, wear of the sealing surfaces on the discs may affect the extent to which seal is adequately provided.